1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method, and more specifically, to a configuration that resolves an uneven density attributed to the difference in print time between passes which may occur during printing based on what is called a multi-pass printing system.
2. Description of the Related Art
As a printing apparatus in a printer, a copy machine, a facsimile machine, or the like or a printing apparatus used as equipment for outputting information processed by composite electronic equipment or a workstation including a computer, a word processor, or the like, an ink jet system-based printing apparatus is popular which can use a relatively simple configuration to print various print medium such as paper, cloths, plastic sheets, and OHP sheets. This system is basically of a non-contact printing type and does not depend on the type of print medium. Accordingly, a printing apparatus has been proposed which uses as print media not only those mentioned above which are normally used but also cloths, leathers, nonwoven cloths, and metals.
Further, such an ink jet printing apparatus mainly employs what is called a serial system that prints an image while scanning a print head in a direction (hereinafter referred to as a main scanning direction) crossing a direction (hereinafter referred to as a sub-scanning direction) in which print media are fed. The serial system-based printing apparatus has the advantage of being able to print images with a relatively high reproducibility and uniformity using a simple configuration. Moreover, as an ink ejecting system for the print head, there is what is called a bubble jet (registered mark) system that utilizes thermal energy generated by an electro-thermal converting element to generate bubbles in inks so that the pressure of the bubbles causes the inks to be ejected. The bubble jet system has widely been used because of its various advantages; ejection openings can be relatively densely arranged and noise generated in association with printing operations is low.
Further, recent ink jet printing apparatuses employ what is called a multi-pass printing system that improves an image grade by scanning the print head in the main scanning direction a number of times while varying ink ejection openings corresponding to the same print area on a print medium to complete printing that print area.
FIG. 1 is a diagram illustrating the multi-pass printing system. This figure shows multi-pass printing with two passes in which printing is completed by two main scans (hereinafter simply referred to as scans). FIG. 1 illustrates the multi-pass printing on the basis of the relationship between the position of a print head H and an area on a print medium P in which an image is printed. In this figure, an image is printed on the print medium P by causing the print head H to eject inks on the basis of print data, while moving the print head H in the main scanning direction (the direction of an arrow X1). Further, every time the print head H executes one scan, the print medium P is fed in the sub-scanning direction (the direction of an arrow Y) by a width (hereinafter referred to as a ½ band width) corresponding to half of the width of an arrangement of ejection openings in the print head H (hereinafter also referred to as the “width of the print head H”). FIG. 1 illustrates that the position of the print medium P is fixed and that the print head H moves in the direction opposite to the sub-scanning direction with respect to the print medium P.
With the multi-pass printing, during the first scan, printing is executed on the basis of print data obtained by thinning, to half, the pixels in image data for one band width corresponding to the width of the print head H. During the second scan, the print data for one band width is subjected to thinning complementary to the thinning for the first scan. Then, printing is executed on the basis of the print data obtained. Accordingly, the print medium P is printed by allowing the print head H to execute two scans each completing the print area for the half bandwidth. Specifically, for the print area on the print medium P which corresponds to the half band width, the first scan executes printing on the basis of the predetermined thinned print data. The second scan executes printing on the basis of the print data obtained by thinning complementary to the thinning in the first scan. In the description below, for two-pass printing, the first scan to complete printing the corresponding area is called a 1/2 pass. The second pass to complete printing the corresponding area is called a 2/2 pass. Further, for the print area on the print medium P which corresponds to the half bandwidth, an area subjected to only 1/2 pass printing is called a 1/2 pass print area. An area subjected to both 1/2 pass printing and 2/2 pass printing to complete the image is called a 2/2 pass print area.
FIG. 2 is a schematic diagram showing an image being printed by a two pass printing operation. During the printing operation, the print area for the half band width located at a trailing end of the print medium in the sub-scanning direction is a 1/2 pass print area. The 1/2 pass printing operation has executed only printing based on the thinned print data. Thus, the 1/2 pass print area has not been completely printed. If a thinning rate is, for example, half, the density of the area is about half of that accomplished on completion.
As is apparent from the description of the two pass printing system, multi-pass printing can basically be equally executed with any number of passes such as in 4 pass printing in which an image is completed by scanning the print head over the same print area on the print medium in the main scanning direction four times or 8 pass printing in which an image is completed by scanning the print head over the same print area on the print medium in the main scanning direction eight times.
However, with the above multi-pass printing system, when the time interval (herein after also referred to as print time difference) between the first scan and second scan executed on the same print area changes, the print density varies depending on the difference in print time. This may lead to the uneven density of the print image (hereinafter also referred to as time difference unevenness).
FIGS. 3A-3D and 4A-4D illustrate how the density becomes uneven depending on the print time difference.
FIGS. 3A-3D schematically illustrate how an ink permeates through and is fixed to a print medium 102 if there is a relatively small print time difference between the 1/2 pass and 2/2 pass of 2 pass printing. As shown in FIG. 3A, an ink droplet 101a is ejected during the 1/2 pass. The ink droplet permeates through the print medium 102 perpendicularly to the surface of the print medium 102 and along the surface as shown in FIG. 3B. Then, coloring materials such as dyes, that is, ink components, are physically and chemically bound to the print medium 102. Then, an ink droplet 101b ejected during the 2/2 pass as shown in FIG. 3C also permeates through the print medium 102 perpendicularly to the surface of the print medium 102 and along the surface as shown in FIG. 3D. However, the ink droplet 101b does not permeate well through or is not fixed well to the area in which the ink droplet 101a has already been fixed. This is because the ink droplet 101a, having already landed to the print medium, is still permeating through the print medium. Thus, the ink droplet 101b, which lands to the print medium later, permeates through the print medium to below the point to which the preceding ink droplet 101a has reached as shown in FIG. 3D. The ink droplet 101b is then fixed in this area. The landing or fixed position of the ink droplet 101b for the second pass, shown in FIGS. 3C and 3D, is misaligned with respect to the impacting or fixed position of the ink droplet 101a for the first pass, shown in FIGS. 3A and 3B. This is because in the multi-pass printing, the ink ejection data for the respective scans are complementary to each other on the basis of a pixel thinning process. A pixel landing to the print medium during the second pass is not the same as that impacting the print medium during the first pass. These pixels are, for example, adjacent to each other. This also applies to FIGS. 4A-4D.
FIGS. 4A-4D schematically illustrate how an ink permeates through and is fixed to the print medium 102 if there is a relatively large print time difference between the 1/2 pass and 2/2 pass of 2 pass printing compare to the case shown in the figure described above. As shown in FIG. 4A, the ink droplet 101a is ejected during the 1/2 pass. As shown in FIG. 4B, the ink droplet permeates through the print medium 102 perpendicularly to the surface of the print medium 102 and along the surface, similarly to the case shown in FIG. 3B. The coloring materials such as dyes, that is, the ink components, are physically and chemically bound to the print medium 102. Then, the ink droplet 101b ejected during the 2/2 pass and landing to the print medium later than the ink droplet 101a as shown in FIG. 4C permeates, at relatively large amount, through the print medium 102 to the area to which the preceding ink droplet 101a has permeated through and has been fixed (FIG. 4D), unlike in the case shown in FIG. 3D. This is because there is a relatively large print time difference between the 1/2 pass and the 2/2 pass, so that the ink droplet 101a, having already landed to the print medium, can sufficiently permeate and spread or its volatile components can be evaporated. As a result, the amount of ink droplet 101a per unit area of the print medium 102 decreases to enable the ink droplet 101b, which lands to the print medium later than the ink droplet 101a, to permeate through this area.
In this manner, the amount of ink fixed to the vicinity of the surface of the print medium, that is, the amount of coloring materials such as dyes, varies depending on the print time difference between the 1/2 pass and the 2/2 pass. The print density corresponds to the quantity of light absorbed by the coloring materials fixed to the vicinity of the surface of the print medium. Accordingly, the print density varies depending on the print time difference between the 1/2 pass and the 2/2 pass.
As described above, if an image is printed using the multi-pass printing system, when there is a difference in the print time difference between the 1/2 pass and the 2/2 pass, the time difference unevenness may occurs between the preceding print image and the current print image.
FIG. 5 is a diagram illustrating the relationship between the width of a print image (also simply referred to as the “print width” herein) that is a length over which the print head is scanned during bidirectional multi-pass printing and the print time difference between the 1/2 pass and the 2/2 pass. The print head H is scanned in the rightward direction of FIG. 5 during the first scan and in the leftward direction during the second scan, that is, bidirectional printing is executed during the first and second scans. Then, focus will be placed on an area A of a predetermined size located at the left end of the print image and an area B of a similar size located at the right end. In the area A at the left end, the 1/2 pass printing is executed as an early stage in which the print head H is scanned rightward. Then, the print head starts to return at the right end and then executes the 2/2 pass printing as a final stage in which the print head is scanned leftward. Thus, there is a relatively large print time difference between the 1/2 pass and the 2/2 pass. On the other hand, in the area B at the right end, the 1/2 pass printing is executed as the final stage in which the print head is scanned rightward. Then, the print head starts to return at the right end and then executes the 2/2 pass printing as the early stage in which the print head is scanned leftward. Thus, there is a relatively small print time difference between the 1/2 pass and the 2/2 pass. In an area such as the area A in which printing is executed in the final scan stage after the print head has started returning, the print time difference corresponds to the print width. Thus, the print time difference varies depending on the print width. As a result, the difference in density between the areas A and B increases consistently with the print width.
FIG. 6 is a diagram illustrating the unevenness of the density of (the unevenness of the time interval for) a print image caused by a variation in the print time difference between the area at the right end and the area at the left end in the scanning of the print head in the bidirectional multi-pass printing. In FIG. 6, an area in which there is a long print time difference between the 1/2 pass printing and the 2/2 pass printing is denoted by α. An area in which there is only a short print time difference between the 1/2 pass printing and the 2/2 pass printing is denoted by B. The figure indicates that in the bidirectional printing, the area α alternates with the area B. The difference in density between these areas appears as a repetition of a high and low densities at the opposite ends of the print image. This is markedly perceived as time difference unevenness.
The above description relates to the 2-pass multi-pass printing. Of course, similar time difference unevenness may occur in multi-pass printing with three passes or more. In these cases, for example, in m (m equal to or greater than three) pass printing, a similar time difference unevenness may result not only from the time interval between a (m−1)/m pass printing operation and a m/m pass printing operation that completes the printing, but also from the time interval between a k/m pass printing operation and a (k+1)/m pass printing operation both performed before the printing is completed, the time interval corresponding to two consecutive scans. That is, the areas which have respectively become the areas α and B during the (k+1)/m pass become the areas B and α, respectively, during the next (k+2)/m pass. However, the above time difference unevenness occurs during the (k+1)/m pass in accordance with the manner in which the ink permeates and is fixed. This density unevenness affects the final time difference unevenness of that area. For multi-pass printing with three or more passes, the final time difference unevenness on the completion of the printing creates a problem, similarly to the time difference unevenness during the scan.
The above time difference unevenness problem is significant in a printing apparatus that prints large-sized sheets. The recent ink jet printing apparatuses can print not only relatively small-sized print media such as an A4 and A3 sizes as in the conventional case but also relatively large-sized print media such as those having a width of 36, 42, 64, or 72 inches. In such a large-sized sheet printing apparatus, the difference in density between the areas A and B corresponds to the print width as described above in FIG. 5. Consequently, there is a larger difference in density between these areas, resulting in the remarkable occurrence of time difference unevenness. Therefore, the time difference unevenness is a particularly serious problem for large-sized sheet ink jet printing apparatuses that can print relatively large-sized sheets.
Japanese Patent Application Laid-open No. 2003-034021 describes a known technique to suppress the time difference unevenness resulting from a print time difference as described above. According to this document, a print area is divided in a plurality of areas in the main scanning direction. Then, the numbers of dots of a black ink and other color inks applied to each area are counted. If there are any areas in which the numbers of dots of the black ink and other color inks exceed the respective thresholds, the number of such areas is counted. When the count has at least a predetermined value, it is determined that a density unevenness (corresponding to the above time difference unevenness) is likely to occur. Thus, a print mode is switched from bidirectional printing to one-directional printing. This makes it possible to suppress the time difference unevenness occurring at the opposite ends of the print medium. This document also contains the description that the width of image data printed, that is, the width of an area scanned by the print head, is detected and that when the width is small, it is determined that the degree of the time difference unevenness is small to avoid switching to the one-directional printing even if the number of areas has at least the predetermined value.
However, the technique to suppress the time difference unevenness, which technique is described in Japanese Patent Application Laid-open No. 2003-034021, basically determines the numbers of ink dots to determine whether or not time difference unevenness occurs in an area in which the numbers of ink dots have at least the predetermined values, so as to switch the scanning direction in accordance with the result of the determination. Thus, the amounts of load and data required for a process of counting the numbers of ink dots correspondingly increase. This results in the need for an extra time for processing. In particular, for an image to be printed on a large-sized sheet ink jet printing apparatus has an enormous amount of print data, the above problem is more serious.